Block or charge depends on position, timing, and path.

How the call is made

1. Check whether the defender got to the spot with legal guarding position before the contact.
2. Judge whether the defender moved laterally or backward legally, or instead slid into the path too late.
3. Identify the point of contact and which player caused the displacement.
4. Ask whether the play is inside a restricted-area rule that limits charge calls on help defenders.
5. If the offense created illegal contact against an established defender, it can be a charge. If the defender was late or out of position, it is more likely a block.

Edge cases fans miss most

• Secondary defenders get less freedom near the rim: restricted-area rules exist to stop very late slide-under plays from being rewarded.
• Torso matters more than dramatic flops: officials focus on the actual displacement, not the reaction after it.
• The defender does not have to be motionless: moving backward or laterally can still be legal once position is established.
• Off-arm and leg extension can change the ruling: even if the main body contact is close, extra extension by the attacker can create the offensive foul.

Position is more than planting both feet

A defender initially establishes legal guarding position by getting in front of the opponent in the manner required by the rule code. After that position exists, the defender is not frozen to the floor. The defender may turn to absorb contact, move laterally or backward to maintain position, and protect against injury. What the defender generally may not do is move forward into the opponent, extend a hip, knee, shoulder, or leg into the path, or arrive after the offensive player's upward motion has begun.

The point of contact matters too. Contact through an established defender's torso supports an offensive-foul ruling more than contact caused by a defender reaching or leaning outside that legal position. Marginal contact can remain a no-call; officials do not have to choose block or charge every time bodies touch.

The restricted area has exceptions

The restricted arc mainly limits charges drawn by secondary defenders near the rim. It does not erase every offensive foul inside the arc. Primary-defender situations, plays beginning in the lower defensive box, unnatural use of an arm, knee, or leg, and a defender jumping vertically to contest can produce different outcomes. Exact exceptions vary between NBA, FIBA, college, and school rules, so the arc should be treated as one step in the decision rather than an automatic blocking foul.

"He was there first"

Being there first helps, but it is not the full test. The defender must still establish legal guarding position and cannot arrive so late that the offensive player has no fair chance to avoid the contact.

Officials also protect an airborne player’s landing opportunity. A defender who moves underneath after the attacker has left the floor can be responsible for the contact even if the defender reaches the landing spot before the attacker comes down. Conversely, an attacker cannot escape responsibility by jumping first and then leading with a knee, foot, or extended arm. The complete sequence—not a freeze-frame immediately before impact—determines the ruling.

What the whistle changes

A charge is an offensive foul and usually gives the ball to the defense. A block is a defensive foul and may mean side-out possession or free throws depending on the shooting status, team-foul count, and competition rules.

Source material

• FIBA Official Basketball Rules
• NBA Rulebook
• NCAA Basketball Rules
• NFHS Basketball Rules